Dolostone, also known as dolomite (not to be confused with the mineral dolomite), is a type of carbonate sedimentary rock consisting predominantly of dolomite [CaMg(CO3)2] and to a lesser extent of other carbonates, like calcite and aragonite. Like the mineral, the name derives from the French mineralogist Déodat Gratet de Dolomieu, after whom also the Dolomites of Italy, where the rock was discovered, take their name. Likewise limestone, dolostone forms as a result of carbonate precipitation from water, in particular dolomite, which precipitates following the reaction:
Ca2+ + Mg2+ + 2(CO3)2- ⇌ CaMg(CO3)2
The precipitation of dolomite from a water solution is controlled by temperature, pressure, and solubility of CO2. Differently from limestone, which is produced by a wide range of organic and inorganic processes that occur at ambient conditions, the precipitation of primary dolomite directly from the water column is very rare and seems not to occur in present-day ocean, despite the fact that seawater is saturated in dolomite. Rather, most dolomitic rocks derive from the chemical replacement and cementation of original calcite and aragonite carbonate sediments by secondary dolomite. The process of dolomitization tends to produce rhombohedral crystals of dolomite, modifying in part or totally the texture of the original limestone. Some dolostones still preserve fossils or other allochems, while others only show a crystalline texture, consisting of recrystallized rhombohedrons of dolomite, which could either represent primary dolomite or completely dolomitized limestone. Dolomitization of carbonate sediments seems to happen mainly by two mechanisms: evaporation of seawater that produces gypsum-rich brines that migrate in the underlying carbonate sediment, reacting with calcite to produce dolomite (evaporative reflux) or interaction between meteoric and marine groundwater (mixing of seawater and meteoric water).
Dolostone o dolomite
Carbonate sedimentary rock
• skeletal grains (fossils)
• grain aggregates
• dololutite, doloarenite, dolorudite…
• dolomudstone, dolowackestone, dolopackstone…
• dolobiomicrite, biosparite…
Recognition of dolostone
Dolostone can be distinguished from limestone e chert using a hardness test and an acid test (reaction with HCl). The reaction of dolostone with a 10% hydrochloric acid (HCl) – water solution is very slow and does not produce the characteristic fizz, which is produced by the reaction between HCl 10% and limestone. A more concentrated solution (30% HCl) can, however, produce an effervescent reaction even in dolostone. Dolomite has hardness around 4 on the Mohs scale and it is, hence, easily lined by a metal rod, whereas chert is stronger than metal (and dolomite) to scratch.
Dolostones commonly produces a conchoidal fracture when hammered or on fresh, broken surfaces, similar to that of other fine-grained crystalline sedimentary rocks like limestone and chert. Moreover, like all carbonate rocks, dolostones are subject to slow dissolution in water at ambient conditions, which produces karstic features on outcrops of dolostones. The mineral dolomite can contain small quantities of iron (thanks to a partial solid solution with ankerite [CaFe(CO3)2]) and, consequently, weathered outcrops of dolostone tends to be covered by a yellowish to reddish (rusty) patina.
The recognition of calcite from dolomite in thin section is more difficult, because these minerals have identical habit. The only difference between calcite and dolomite is the orientation of glide twins, which, unfortunately, are rare in undeformed dolomite crystals. The distinction becomes straightforward by treating thin sections with alizarin red, which stains calcite and leaves dolomite unstained, as in the example below:
Classification of dolostones
Most dolostones contain calcite or aragonite along with dolomite, because the dolomitization process is often incomplete. This allows to classify dolostones, based on the content of dolomite with respect to calcite.
The precise estimation of the proportion of calcite and dolomite in a carbonate rock can be done using alizarin red (see above). In the field, this classification can be done qualitatively by observing the effervescence of the reaction, which is maximum in limestone and decreases with intensity with increasing dolomite content.
Although useful to understand the composition of dolostone, the classification scheme above does not provide any information on the texture of the rock. If a dolostone preserves, even if partially, the texture of the original limestone (e.g. allochems, micrite, spar), it is possible to classify it using the same classification schemes used for limestones (e.g. Grabau, Dunham, Folk, Embry & Klovan; more details here), adding the dolo- prefix to the rock name obtained using these classification schemes. For example a dolomite with ooids and cement can be classified as a doloarenite according to Grabau, a dolograinstone according to Dunham, and a dolooosparite according to Folk.
Examples of dolostone
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